This invention relates generally to an apparatus for generating chlorine dioxide. In one aspect, it relates to a solid-state chlorine dioxide generator. In another aspect, it relates to a chlorine dioxide generator featuring a novel reaction column which provides exceptional reaction efficiency and allows the generator to operate over a wide range of production rates.
Chlorine dioxide (ClO2) is a strong oxidizing agent and is used in a variety of industrial applications including municipal water treatment as a bactericide, taste and odor control, and zebra mussel infestation control, to name a few. ClO2 is a powerful viricide, bactericide, protocide, and algaecide. In addition, ClO2 does not form THM""s (trihalomethane) which is a carcinogen. ClO2 can be prepared by oxidation of chlorites or reduction of chlorates. In a preferred embodiment, the generator of the present invention is a vacuum-driven sodium chlorite (NaClO2)/chlorine (Cl2) reactor. The Cl2 may be used directly or it may be formed by the reaction of HCl and sodium hypochlorite.
At normal operating pressures and temperatures, ClO2 is a gas and is extremely explosive (above about 300 mm Hg pressure ClO2 may detonate). Because of its explosiveness, ClO2 is usually generated under a vacuum and dissolved in water for use. ClO2 is unstable chemically and thus cannot be shipped; it must be generated on site.
A number of vacuum-driven ClO2 generators are commercially available. Vulcan Chemical Co. (formerly Rio Linda Chemical Co.) markets a ClO2 generator under the trade designation xe2x80x9cWM-Seriesxe2x80x9d. This generator is similar to the generator disclosed in. U.S. Pat. No. 4,590,057. As seen in FIGS. 1 and 2 of U.S. Pat. No. 4,590,057, the Vulcan generator is an assembly of pieces of tubing interconnected by fittings, couplings, elbows, external valves and the like. These components are secured to a relatively large mounting board or surface. The several connections required present sources of pressure and vacuum leaks which could result in poor performance and conversion efficiency. Other disadvantages include higher maintenance and increased space requirements for installing the generator at the industrial site. The conversion efficiency is defined as the ratio of the actual amount (mass) of ClO2 produced to the theoretical limit for a given amount of reactants (see Equation (1) below).
As described in detail below, the preferred generator of the present invention is a solid-state generator with feed lines and check valves bored into and incorporated in a solid block. The present generator has low maintenance and is compact requiring a small installation area.
The prior art vacuum ClO2 generators all have a reaction column where the reactants are brought together and react to form ClO2. The Vulcan generator employs a reaction column filled with particulate packing to enhance mixing of the reactants in the reaction column. Without the packing, the reactants may by-bass one another within the reaction column and thereby reduce efficiency. The use of packing is objectionable because i) it causes a pressure drop across the reaction column which limits throughput, and ii) it may become clogged over time requiring shutdown of the generator for replacing and/or cleaning the packing and reaction column.
The present invention provides a vacuum-driven ClO2 generator that i) has solid-state construction and/or ii) employs a novel xe2x80x9ctunedxe2x80x9d reaction column that does not require a packing material and yields excellent ClO2 production efficiency.
The present generator comprises a body having formed therein a tuned reaction column wherein the reactants combine to form ClO2 gas, an eductor connected to the reaction column for dissolving the ClO2 gas in a water stream to form an aqueous ClO2 solution, and a discharge line for discharging the solution. The reactant inlets, reaction column, eductor, and discharge are preferably interconnected through passages bored or machined in the body thereby eliminating the assortment of hardware (i.e. fittings, elbows, etc.) used in prior art designs. The solid-state design of the present generator eliminates pressure and vacuum leaks, reduces maintenance, and reduces the overall size of the generator for convenient installation on site.
The preferred reactants are an aqueous solution of sodium chlorite (NaClO2) and chlorine gas (Cl2). Other reactants such as HCl and NaOCl (to generate Cl2), may also be used. The eductor comprises a suction chamber which is fluid communication with the reaction column. Suction pressure is established in the chamber using a water nozzle wherein the water velocity increases while the pressure decreases (according to Bernoulli""s principle) thereby creating a suction pressure. The suction draws the reactants through the generator inlets and into the reaction column wherein the conversion of the reactants to ClO2 occurs almost instantaneously. The ClO2 gas is then sucked into the water stream where it dissolves to form an aqueous solution of ClO2. The concentration of the chlorine dioxide aqueous solution is controlled by controlling the reactant flow rates into the generator. The present generator is capable of producing an aqueous chlorine dioxide having a concentration in the range of 200 to 3300 mg/liter.
In this reaction, each mole of Cl2 gas consumed yields two moles of ClO2 gas product (see Equation (1) below). Thus the products of the reaction will have a larger volume (approximately two times or more) than the reactants. The reaction column is xe2x80x9ctunedxe2x80x9d to accommodate this increase in volume by providing a segment of increasing flow area in the form of a flow expander (i.e. a diffuser) wherein the increasing flow area in the direction of flow i) accommodates the increase in gas volume, ii) maintains the reactants at nearly constant velocity whereby one reactant will not flow past another unreacted, and iii) induces some turbulent mixing which acts to promote the reaction. This tuning has the effect of allowing the reactants to thoroughly mix thereby improving the conversion efficiency of the generator.
By the time the flow reaches the outlet of the diffuser segment of the reaction column, the reaction is complete and the volume of the products thereafter remains constant. The diffuser segment discharges the products (ClO2 and NaCl) into a cylindrical column of constant flow area wherein the velocity of the ClO2 gas is virtually constant. This insures a steady constant velocity flow of ClO2 into the eductor wherein the ClO2 is dissolved in water forming an aqueous ClO2 solution. The tuned reaction column allows the unit to operate efficiently at lower ClO2 production rates without variations in meter readings and with a greater xe2x80x9cturndownxe2x80x9d ratio. ClO2 is generated at a minimum of 95% efficiency with no more than 5% excess Cl2 and/or NaClO2. The ClO2 solution is discharged from the generator outlet and conducted by tubing to the point of application.
The turndown ratio is defined as the percent of maximum ClO2 production rate. For example, a generator having a maximum production rate of 500 lbs of ClO2 per day, operating at a rate of 50 lbs ClO2 per day would have a 10% turndown ratio. The present generator has been shown to provide excellent conversion efficiency at turndown ratios as low as 10%. Prior art generators are limited to turndown ratios of about 30% and above. At ratios below 30%, these generators exhibit degradation in conversion efficiency due to inadequate mixing of reactants. Poor conversion efficiencies can lead to uncertainties in the ClO2 production rate and ClO2 concentration levels in the applied aqueous solution.